Design Flaw in Deutsche Telekom Speedport w700v broadband router

Hi,
I'd like to inform you about a vulnerability in the Deutsche Telekom Speedport w700v DSL router. Currently it's the standard device that is shipped with new DSL contracts.

I - TITLE

Security advisory: Weaknesses in the login process of the web interface
of the Speedport w700v DSL Router and Wireless LAN
Access Point

II - SUMMARY

Description: A design flaw exists in the login process of the web interface
of the Speedport w700v DLS Router and Wireless LAN Access Point
of Deutsche Telekom that might lead to unauthorized access.

Author: Michael Domberg (mdomberg at gmx dot li)

Date: May 11th 2007

Severity: Medium

References: http://www.devtarget.org/speedport700-advisory-05-2007.txt

III - OVERVIEW

The Speedport w700v is an ADSL/ADSL+ broadband router, Wireless LAN Access Point,
4-Port-Switch and telephone system with integrated firewall and advanced security
features.

More information about the product can be found online at
http://www.t-com.de

IV - DETAILS

The Speedport firmware consists of some CGI-Scripts that interact with the
hardware and some static html-pages as front-end. The login to the web
interface is designed the same way.
Upon submitting the system password (no username required…) the password
is sent to a cgi-script that verifies the password with internal sources. If
the verification is successful, the welcome screen of the interface is returned.
If the verification failed the login screen is returned. To avoid brute force
attacks, the login page contains some JavaScript that disables the input field
for a certain amount of seconds. The first attempt is one second delayed, the
second is two second delayed and any further attempt is delayed for the doubled
amount of time of the previous one. So the 8th attempt requires the attacker to
wait for about 4 minutes.
By submitting the request directly to the underlying cgi-script and verifying the
result page an attacker can circumvent this mechanism and perform multi-threaded
brute-force attacks.

V - ANALYSIS

The severity of this vulnerability is to be considered "medium". The default password
of the web interface is "0000". So users often choose a four-digit numeric password, too.
The Speedport 700 series is one of the most-sold DSL modems, because it is the standard
hardware for german DSL users of Deutsche Telekom.
Users can prevent their modems from being exploited this way by disabling remote
administration access (which is the default).

VI - EXPLOIT CODE

An PoC is available, but not published.

VII - WORKAROUND/FIX

Users have to disable remote administration access to prevent their routers from being
exploited.
The vendor doesn't seem to address this vulnerability.

VIII - DISCLOSURE TIMELINE

22. February 2007 - Notified vendor of affected software
23. February 2007 - Vulnerability confirmed
24. May 2007 - Public disclosure

Regards,
Michael Domberg,
www.devtarget.org